Recently, as concerns for the environment are gradually rising, needs for low noise refrigerators are increasing. To this end, a noise level of a refrigerator may be reduced, or freezing capacity of a refrigerator may be adjusted according to whether the refrigerator operates in the daytime or the nighttime.
Refrigerators including a linear compressor as a type of reciprocating compressor are recently commercialized. Such a linear compressor applied to refrigerators is controlled to operate according to loads by varying only a stroke of the compressor without varying a frequency thereof. Since the frequency is an important factor, the frequency is determined based on mechanical characteristics such as the characteristics of a spring disposed in the compressor, required freezing capacity, optimized efficiency, and noise characteristics.
Since a linear compressor has optimal efficiency at a resonant frequency, its operation frequency is controlled to correspond to the resonant frequency. A stroke of a piston of the compressor is increased with the operation frequency approaching the resonant frequency, so that a flow rate of refrigerant is adjusted to match with freezing capacity corresponding to a load to the refrigerator.
The resonant frequency is determined by a modulus of elasticity of a mechanical spring in the compressor; a modulus of elasticity of an injected gas spring; and the mass of both the linearly reciprocating piston and a member connected to the piston. The refrigerant in a compressed space uses its own elastic force to function as the gas spring, thereby elastically supporting the piston.
In general, when a compressor is manufactured, its resonant frequency, that is, a frequency having the maximum efficiency is determined. In addition, a frequency having the optimal noise level is determined. Furthermore, each of the resonant frequency and the frequency having the optimal noise level may be determined in plurality.
To satisfy consumers with respect to a noise from a refrigerator, the number of rotations of a compressor as a main noise source of the refrigerator may be varied such that a sound quality index related to an ambient noise during the operating of the refrigerator follows a sound quality index related to an ambient noise during the stopping of the refrigerator.
However, in this case, since a periodic noise variation according to various operation conditions such as starting of the compressor and driving of a refrigerating compartment and a freezing compartment is measured to calculate a sound quality index, a frequent noise variation for following the sound quality index may annoy a consumer.
In addition, since only the varying of the number of rotations of the compressor is insufficient to satisfy a sound quality index related to a noise varied according to operation states of the refrigerator, the number of rotations of the compressor may be excessively reduced to thereby degrade the performance of the compressor.
In addition, even when the number of rotations of the compressor is reduced based on a sound quality index to reduce a noise, the number of rotations of a refrigerator compartment fan and the number of rotations of a freezing compartment fan should be increased to prevent a performance degradation of the refrigerator, thereby further increasing a noise.